Question

In my unit test I need to mock an interface which among different methods has nextItem() and isEmpty() methods:

public interface MyQueue {
    Item nextItem();
    boolean isEmpty();
    //other methods
    ...
}

My requirement for the mock is that isEmpty() initially should return false, but after nextItem() was called isEmpty() should return true. Thus I’m mocking a queue with one item.

1. What is the simplest way to implement this kind of mock with mockito?
2. Can I implement additional requirement: calling nextItem() second, third time and so on will result in a specific kind of exception?

P.S. I don’t want to provide the full implementation of my interface for the test, because of other methods in it, resulting in hard-to-understand and verbose code.

Answer 1

You can achieve that with thenAnswer(), a feature Mockito documentation sees as controversial:

Yet another controversial feature which was not included in Mockito originally. We recommend using simple stubbing with toReturn() or toThrow() only. Those two should be just enough to test/test-drive any clean & simple code.

Here’s thenAnswer:

private boolean called = false;

when(mock.nextItem()).thenAnswer(new Answer() {
 Object answer(InvocationOnMock invocation) {   
     called = true;       
     return item;
 }
when(mock.isEmpty()).thenAnswer(new Answer() {
 Object answer(InvocationOnMock invocation) {          
     return called;
 }
});